Some substrates have three dimensional surfaces where imaging is challenging. For example, in the gravure printing industry a unique image carrier, which is usually made of a steel or aluminum based cylinder is used. The cylinder is electroplated with copper and then etched chemically or engraved electromechanically such that print areas are below the surface of the cylinder. In the image areas, there are thousands of micro size cells to be etched or engraved into the cylinder.
In the chemical etching process, the cylinder is coated with a light sensitive emulsion of photoresist. A photo graphic art work film is placed in direct contact with the photoresist coating. The photoresist is then exposed with ultraviolet light projected through the photo graphic film. The cylinder is then developed with water or water based chemicals, rinsed from the surface to carry away the unexposed material leaving a rigid, hard-edged resist. The cells develop cleanly and leave exposed copper metal where the resist is removed. The next step is a chemical etching process to etch away the exposed copper. Although the process is widely used in imaging gravure cylinders, it is tedious, expensive and wasteful of materials.
Another process, electromechanical engraving, is very popular today. The physical process of cutting the cells is done on a specially built metal lathe designed to cut a fixed pattern of very small diamond shaped holes into a copper cylinder in a uniform manner. An engraving head controls the pattern of cells and the cell size. High quality bearings and a unique drive mechanism on the bed of the engraver control the uniform motions of the cylinder's rotation and the lateral movement of the engraving head carriage. However, the processing times for cylinders using this technique can last for days depending on the combination of cylinder size and cell size.
Laser engraving is another method used to image the cylinder where computer-directed lasers, which, like the electromechanical method, cut cells of varying depths and sizes. The original is scanned into a computer, the various image densities are determined, and lasers etch the cylinder. This method has limitations due to the difficulty in forming images due to the high light reflectance of copper and the complexity of the process.
In the past, printing techniques have also been proposed and put in practical use for forming images on three dimensional (3D) objects such as the cylinders. Of these printing methods, ink jet printing was preferred owing to its high speed printing turnaround and little waste generation. Two common methods used to form images on cylindrical objects using inkjet printing in the past were direct printing method and transfer method.
In direct printing method inkjet printing on 3D or cylindrical objects can be achieved either by moving the printhead or the object; however, the stationary printhead configuration limits the size and shape of the objects to be moved conveniently under the printheads. In the case of moving printheads, the printheads need to be moved over the object, or the object under the printheads, such that a path following the shape of the object is traced out. The challenge here is to find an accurate robot motion system that can move the printhead with sufficient accuracy and repeatability to allow images or multiple dots, i.e. print swathes, to be aligned. Most robot systems define an accuracy with which they can move to a certain point in three dimensions rather than the accuracy and uniformity with which they can trace the path required for printing.
In transfer method an image is printed on an intermediate medium such as paper and is then transferred onto a final substrate using heat, pressure and/or by using release layers. The imaged transfer paper is placed over the cylindrical object with the image facing down. Then, the back surface of the paper is heated with a hand iron or pressed to release the image from the paper onto the cylinder. After completely transferring the image onto the cylinder, the paper is removed. In some cases, additional coatings called release layers are coated such that they can easily be peeled off leaving the image on the final substrates. The weakness of this approach is that the use of heat and pressure on the ink affects the image resolution.
Although there a numerous processes for forming images on three dimensional objects, there is still a need for a method which substantially reduces or eliminates the problems of forming images on three dimensional substrates as mentioned above.